This invention relates to a loudspeaker vibrating diaphragm and methods for manufacturing the same.
A material for use as a vibrating diaphragm in a loudspeaker is required to have a low density, a high Young's modulus, a high rigidity and an appropriate high internal loss.
A vibrating diaphragm made of an olefin resin is often used in a loudspeaker since it not only has an excellent water resistance and other environment resisting properties, but also has a good outside appearance and a good balance among various physical properties.
However, since the olefin resin has a specific gravity of 0.9 g/cm.sup.3 which is higher than that of paper, a loudspeaker vibrating diaphragm made of the olefin resin has a low Young's modulus and a low rigidity.
Recently, in order to improve the intensity of a loudspeaker vibrating diaphragm, a carbon fiber has been suggested to be used as a filler to obtain the desired intensity. This however causes a further increase in density, resulting in a low sensitivity and causing difficulty when a sound having a high frequency is to be produced.
In order to solve the above problems, there has been suggested a method as shown in FIGS. 14(a) and 14(b). As illustrated in FIG. 14(a), a sheet 50 which is a foam material made of a polypropylene, is heated so as to become soft. Then, as illustrated in FIG. 14(b), the sheet 50 is formed into a vibrating diaphragm having a predetermined shape, by means of a metallic mold 23 and through a vacuum treatment. Besides, there is another method where a sheet which is the same as that in FIG. 14 is treated so as to form a desired shape in a heating/cooling pressing process.
However, a vibrating diaphragm produced in the above-related methods has a bad outside appearance since its foam layer is exposed. Moreover, since polypropylene material is exposed like a thin film to the surfaces of the sheet 50, the diaphragm itself is poor in its light resistance (for instance, ultraviolet rays).
In view of above, there has further been suggested an injection molding method where a vibrating diaphragm is directly formed with the use of an injection molding machine. In the injection molding method, a predetermined amount of resin containing a foaming agent is injected into a metallic mold, and a desired foaming process is caused to occur within the metallic mold. In order that the foaming agent can smoothly foam, a necessary space is required to form within the metallic mold. Usually, such a space for a foaming process is formed by first injecting a nitrogen gas into the metallic mold and then discharging the nitrogen gas. Another method of forming a necessary space for foaming is to inject a resin material into the cavity of a metallic mold in a manner called short-shot.
However, when a nitrogen gas is used to form a necessary space for foaming, the thickness of the resin injected in the metallic mold must be 4-5 mm or more, otherwise it will be difficult to achieve the desired foaming effect. On the other hand, if the necessary space for foaming is to be formed by injecting the resin material in the manner called short-shot, the thickness of the resin injected in the metallic mold must be 2-3 mm or more, otherwise foaming hardly occurs.
As a result, the injection molding methods, as above mentioned, can only be used to manufacture a product having a thickness of 2-3 mm or more. In fact, none of any known methods can be used to produce a thin foam product (preferably having a thickness of about 0.2-0.5 mm ) which can be used as a vibrating diaphragm in a loudspeaker.